This invention pertains generally to the field of optical measurements and measuring instruments, and, more particularly, to reflectometers for measuring the characteristics of optical systems, subsystems, and components. Specifically, the invention is directed to a method and apparatus for calibrating an optical measuring instrument in the form of a polarization independent optical coherence domain reflectometer having a polarization diversity receiver, or any optical system that incorporates such a receiver, including an optical telecommunications system. More broadly, the calibration method and apparatus in accordance with the principles of the invention can be applied to calibrate any optical coherence domain reflectometer.
The traditional technique for performing reflectometer measurements is known as optical time domain reflectometry (OTDR). This procedure is useful in the manufacture, installation, and maintenance of optical fiber systems. Briefly, the approach comprises injecting a short intense pulse of light into a fiber and measuring the time-dependent backscattering light signal. This measured signal contains information about the location and magnitude of discontinuities, defects, and anomalies of the fiber and other factors which affect light propagation, such as temperature, mechanical strain, and electric and magnetic fields. A review of this technology has been written by Peter Healey in an article entitled "Review of Long Wavelength Single-Mode Optical Fiber Reflectometry Techniques," published in the Journal of Lightwave Technology, Vol. LT-3, No. 4, August 1985, pp. 876-886.
The conventional OTDR technique becomes less useful when it is applied to small optical systems because of the limits on the measurement resolution inherent in this procedure. Typically, the resolution obtained with an OTDR measurement is of the order of 10 meters, and, in practice, the resolution limit of this approach is approximately 1 meter. See, "OFDR Diagnostics for Fibre and Integrated-Optic Systems," S. A. Kingsley and D. E. N. Davies, Electronics Letters, Vol. 21, No. 10, March 1985, pp. 434-435. Clearly, the conventional OTDR technique is not useful in analyzing small optical systems such as integrated optic circuits, or for high-resolution fiber-optic sensing such as measuring stresses at short intervals along an optical fiber.
Improved resolution can be obtained by means of a technique termed "optical frequency domain reflectometry" (OFDR), or also commonly referred to as FMCW (frequency modulated continuous wave) reflectometry. This procedure is described in the above-referenced article by Kingsley and Davies and in the paper entitled "Optical Frequency Domain Reflectometry in Single-Mode Fiber," written by W. Eickhoff and R. Ulrich, published in Applied Physics Letters 39 (9), 1 November 1981, pp. 693-695. This approach comprises injecting a highly monochromatic beam of light into an optical system, varying the frequency slowly with a time-linear sweep, and detecting the backscattered signal. Detection is achieved by the heterodyne technique, in that the backscattered signal is mixed coherently with the reference input signal. The beat frequency is measured and yields the position of a reflection point in the optical system. The amplitude of the beat signal also determines the backscattering factor and attenuation factor for the reflected light. The article by Kingsley and Davies, cited above, reports a resolution of about 3 millimeters obtained by this technique and estimates that this can be improved to approximately 1 mm with existing technology.
Clearly, the OFDR technique offers the capability of improved resolution compared to the conventional OTDR procedure. Since the OFDR approach is a coherent measurement of interference between a backscattering signal and a reference signal, it also offers a greater dynamic range and improved signal-to-noise ratio over a standard OTDR measurement of reflected signal power. Since the OFDR technique requires only low optical input signal power, the nonlinear effects of optical transmission in the optical fiber are reduced. However, there are also certain drawbacks to the OFDR procedure. Not only does the approach require a highly monochromatic source, but it is also sensitive to frequency-sweep nonlinearities, and it is limited by the frequency sweep range.
It is also noted that heterodyne detection has been used in OTDR systems with very short pulses to achieve resolutions in the micrometer range. Systems of this type are described in a paper by R. P. Novak, H. H. Gilgen, and R. P. Salathe, entitled "Investigation of Optical Components in Micrometer Range Using an OTDR System With the Balanced Heterodyne Detection," and a paper by P. Beaud, J. Schutz, W. Hodel, H. P. Weber, H. H. Gilgen, and R. P. Salathe, entitled "High Resolution Optical Time Domain Reflectometry for the Investigation of Integrated Optical Devices," published in the IEEE Journal of Lightwave Technology, Vol. 25 (1989), pp. 755-759. For purposes of clarity, this technique may be termed "coherent OTDR." These authors report that by using ultrashort pulses and a coherent detection scheme, the OTDR technique can achieve resolutions of about 60 .mu.m in air.
Further improvement in resolution has been obtained by another technique known as "optical coherence domain reflectometry" (OCDR). This procedure is described in the following three articles: (1) "Optical Coherence-Domain Reflectometry: A New Optical Evaluation Technique," by Robert C. Youngquist, Sally Carr, and D. E. N. Davies, Optics Letters, Vol. 12, No. 3, March 1987, pp. 158-160; (2) "New Measurement System for Fault Location in Optical Waveguide Devices Based on an Interferometric Technique," K. Takada, I. Yokohama, K. Chida, and J. Noda, Applied Optics, Vol. 26, No. 9, 1 May 1987, pp. 1603-1606; and (3) "Guided-Wave Reflectometry with Micrometer Resolution," B. L. Danielson and C. D. Whittenberg, Applied Optics, Vol, 26, No. 14, 15 July 1987, pp. 2836-2842. The OCDR approach differs from the coherent OTDR technique in that, instead of a pulsed light source, a broadband continuous-wave source with a short coherence length is used. The source beam enters an interferometer in which one arm has a movable mirror, with the reflected light from this mirror providing a reference beam, and the other arm contains the optical system being tested. The interference signal in the coherently mixed reflected light from the two arms is detected by a heterodyne detection technique and yields the desired information about the optical system.
In essence, the OCDR procedure replaces the beam pulses by the "coherent domains" in a broadband continuous beam, where a domain is defined as a section of the beam in which the light phases are coherently related. The average size of these sections is termed the "coherence length," 1.sub.c, and is of the order 1.sub.c .about.c/.DELTA..nu., where c is the speed of light and .DELTA..nu. is the frequency spread of the light source. See, "Principles of Optics," 4th Edition, M. Born and E. Wolf, Pergamon Press, New York (1970), Section 7.5.8. The heterodyne detection of the backscattered "domains" is accomplished by the technique of "white light interferometry," in which the beam is split into the two arms of an interferometer, reflected by the adjustable mirror and the backscattering site, and coherently recombined. This procedure employs the fact that interference fringes will appear in the recombined beam only when the difference in the optical path length between the two arms is less than the coherence length of the beam, 1.sub.c. The OCDR systems described in references (1) and (3), above, employ this principle, and reference (3) shows interferograms of fiber gaps in test systems obtained by scanning the adjustable mirror and measuring the strength of the recombined signal. Reference (1) also describes a modified approach in which the mirror in the reference arm oscillates at a controlled frequency and amplitude, causing a time-varying Doppler shift in the reference signal, and the recombined signal is fed into a filtering circuit to detect the beat frequency signal.
Another variation of the OCDR technique is illustrated in reference (2), above, in which the reference arm mirror is at a fixed position, and the difference in optical path lengths in the two arms may exceed the coherence length. The combined signal is then introduced into a second Michelson interferometer with two mirrors, one fixed in position and the other being movable. This movable mirror is scanned, and the difference in path length between the arms of the second interferometer compensates for the delay between the backscattered and reference signals at discrete positions of the mirror corresponding to the scattering sites. In practice, an oscillating phase variation at a definite frequency is imposed on the signal from the backscattering site by means of a piezoelectric transducer modulator (PZT) in the optical fiber leading to this site. The output signal from the Michelson interferometer is fed to a lock-in amplifier, which detects the beat frequency signal arising from both the PZT modulation and the Doppler shift caused by the motion of the scanning mirror. This procedure has been used to measure irregularities in glass waveguides with a resolution as short as 15 .mu.m. See, "Characterization of Silica-Based Waveguides with an Interferometric Optical Time-Domain Reflectometry System Using a 1.3-.mu.m-Wavelength Superluminescent Diode," K. Takada, N. Takato, J. Noda, and Y. Noguchi, Optics Letters, Vol. 14, No. 13, 1 July 1989, pp. 706-708.
In short, the OCDR approach offers the capability of high-resolution measurement of optical systems, together with all of the other advantages of coherent reflectometry. The optical dynamic range obtainable with this technique can exceed 100 dB on the power logarithmic scale, which implies that refractive index discontinuities of 10.sup.-5 producing reflected light of the order of 1 femtowatt can be detected. The fundamental limitation on the resolution is the coherence length of the light source, which can be reduced to a few micrometers, with a corresponding increase in source bandwidth.
To summarize the technological state of the art, it is known that improved resolution and signal-to-noise ratios in reflectometry systems can be obtained by using coherent detection schemes; that is, optical interferometry systems in which the reflection signal is coherently mixed with a reference signal and the resulting interference signal is detected. Furthermore, the optimal coherent detection scheme from the standpoint of resolution is the OCDR, in which the resolution is determined by the coherence length of the light source. This resolution can be made very small by using a broadband source.
However, the OCDR, OFDR, and coherent OTDR techniques all share a common problem arising from dependence on the polarization properties of the light beams. This problem arises from the fact that interference between two beams of light can only occur when both beams have the same polarization state. More precisely, the interference signal of two light beams is the incoherent sum of the interference signals from the beam components in two orthogonal polarization states. For example, if one beam is linearly polarized in the horizontal direction and the other beam is linearly polarized in the vertical direction, no interference will occur. Ideally, when the entering beam is split into the two arms of an interferometer, and reflected and coherently recombined, the beam polarization is unchanged. Reference (2), above, includes a polarizer and analyzer mutually aligned at the entrance and exit fibers of the first interferometer to provide this constraint. In practice, any real optical fiber will cause a certain amount of distortion of the polarization of the light propagating therethrough. Changes in the polarization of signals in one arm of the interferometer, or uncorrelated changes in both arms, will degrade the resulting interference signal. Furthermore, this polarization distortion may be time-dependent. Polarization noise and cross-talk in a fiber may be caused by internal and external perturbations from mechanical, thermal, and electromagnetic effects, and can produce fading or reduced visibility of the observed fringes in an interferogram. In addition, the signature of a given backscattering site can be complicated by the group delay differences between two polarization eigenmodes in a birefringent fiber.
In the reflectometry situation, one can eliminate part of the polarization stability problem by careful design and fabrication of the interferometer. Polarization-maintaining fibers can be used, and the measuring instrument can be encased in a housing to substantially insulate it from environmental perturbations. This is only a partial solution because in operation the instrument must be connected to the device being tested, presumably through optical fibers or other transmission means, and polarization distortion may occur in these external fibers or signal conduits. Furthermore, the device under test may itself produce variations in the polarization of the reflectometry signal at reflection or refraction sites or in the optical conduits within the test device. These perturbations may be environmental in origin and may fluctuate with time in an essentially uncontrollable manner. Therefore, in an optical coherence domain reflectometer there is always a polarization instability problem with respect to the optical signals being transmitted and received in the arm of the interferometer that is connected to the device under test.
There is also an additional shortcoming with respect to the above-mentioned references. None of these references or other works in the published literature addresses calibration of an optical coherence domain reflectometer to enable correction of measurements to eliminate the effects of polarization distortion.
A coherent optical reflectometry system which overcomes the problem of polarization variations and distortions during measurements on optical fibers and system components is described in copending U. S. patent application Ser. No. 07/610,188, filed on Nov. 7, 1990, assigned to the same assignee as this application, the disclosure of which is hereby incorporated by reference in its entirety. In particular, in one embodiment, an optical coherence domain reflectometer is disclosed having a light-emitting diode (LED) to provide a broadband source of unpolarized light, and an interferometer in which one arm contains the optical system or device under test (DUT) and the other arm contains a scanning mirror to provide the reference light beam. The reflectometer further includes a polarization diversity receiver (PDR) into which the coherently recombined reflected light is directed. The scanning mirror is driven at a fixed speed to provide a Doppler-shifted reference light signal.
The reference arm also contains a rotationally adjustable polarizer so that the reflected reference signal is linearly polarized. Further fine control of this polarization may be provided by a polarization controller in the DUT arm of the interferometer. The polarization axis may be rotated about the optical axis to calibrate the reflectometer so that equal reference beam powers are provided in the two orthogonal polarization detector circuits of the PDR.
In an alternative embodiment, the light source is an LED followed by a polarizer which provides polarized light. In this version, the linear polarizer in the reference beam arm is replaced by an adjustable birefringent element such as a waveplate or polarization controller. In this alternative embodiment, the reflectometer is calibrated by adjusting the birefringent element to balance the reference beam powers in the detector circuits of the PDR.
Further alternative versions of the reflectometer include additional polarization controllers in the interferometer reference arm or the beam output channel, or both. These polarization controllers allow further fine tuning adjustment and calibration of the reflectometer to compensate for perturbations in the system that affect the light polarization.
In actual use, the reflectometer is first calibrated so that the reference signal E.sub.r (t) has equal components of horizontal and vertical linear polarization, so that equal reference beam power is directed to each branch of the PDR. This is performed by first disconnecting the DUT, so that the reflection coefficient R is negligible, and then rotating a linear polarizer incorporated into the reference arm.
However, the calibration technique disclosed in aforementioned U. S. patent application Ser. No. 07/610,188 has various shortcomings. Central to the operation of the PDR is knowledge of how the optical fields of the reference beam are split between two photodetectors. Each photodetector, however, produces a photocurrent proportional to the optical power, not the optical field, incident thereon. Even though it is possible to deduce the split of the fields from the split of the powers, such deduction requires detailed knowledge of all the proportionality constants. A further complication is that, typically, the photocurrent is not directly measured, but amplified into a voltage signal by a trans-impedance amplifier, which may be followed by other signal processing electronics. The gain of each of these electronic stages becomes another proportionality constant to be characterized in order to convert the power into the field. Therefore, fully calibrated measurements are not achievable.
Thus, there is a need for a simple and effective calibration procedure that removes the uncertainties in the combined proportionality constants in an optical coherence domain reflectometer. It would also be highly desirable to remove uncertainties in the electronic gains in the reflectometer receiver to thereby enhance the accuracy of the measuring instrument.